1. Field of the Invention
The present invention relates to a method of driving a display device which displays a picture of image data by alternating-current driving (referred to as "AC-driving" hereinafter) a display material such as liquid crystals with A.C. power supply, using an active matrix which comprises switching elements such as thin film transistors (referred to as "TFT" hereinafter) and pixel electrodes, the switching elements and pixel electrodes being arranged in a form of a matrix.
2. Description of the Prior Art
Recently, quality of images displayed on an active matrix liquid crystal display (referred to as "LCD" hereinafter) has been remarkably improved to a degree comparable to that of a cathode ray tube (CRT). However, firstly, with regard to image quality, when considering the viewpoints of flickers, changes in brightness in a vertical direction of a screen (i.e., gradient brightness), tone representation and an image memory phenomenon in which a previous fixation image remains as if it is printed just after another fixation image has been displayed, the image quality of an active matrix LCD is not so improved as that of the CRT.
Several attempts have been made to provide a method capable of compensating D.C. voltage that is unavoidably generated in a display device due to dielectric anisotropy of a liquid crystal, reducing occurrence of flicker, thereby increasing the reliability of the driving performance. Two known methods as described above are disclosed in the following materials. One disclosure is "Japan Display" issued by T. Yanagisawa et al. in 1986 (p.192).
In this conventional method, the inevitably generated D.C. voltage in the display device is compensated by changing the amplitude of the positive and negative sides with respect to the amplitude center voltage (Vc) of an image signal voltage (Vs). In this conventional driving method, a scan signal exerts an effect on a potential of a pixel electrode through a parasitic capacitance Cgd existing between a gate and a drain of a TFT, resulting in causing a direct current (D.C.) potential difference between the average potential of a wiring for image signals and the average potential of the pixel electrode. When driving an LC material with A.C. power supply, if a potential at each part in the display device is set such that the average D.C. potential difference between the pixel electrode and the counter electrode becomes zero, the aforesaid D.C. potential difference will be raised inevitably between the wiring for image signals (referred to as "image signal line" hereinafter) and the counter electrode. The raised D.C. potential difference causes serious display defects such as the image memory phenomenon.
A method of compensating a D.C. potential difference completely and making it to be zero is disclosed in "Euro Display" issued by K. Suzuki in 1987 (P.107). In this second conventional method, the compensation of the D.C. potential difference is executed by applying a negative additional voltage signal (Ve) after applying a scan signal. LCDs have normally a feature of a small amount of power consumption for driving, however, in this conventional method, the amplitude of the analog signal is large so that a large amount of power consumption is required in the driving circuit (a degree of several hundreds of milli watts). Obviously, the power consumption required in this method is too large for portable-type display devices operated by dry batteries or the like.
With such a background, the present inventors have disclosed a method of "capacitive-coupling drive" in the Japanese Patent Application Laid Open No. 2-157815, published on June 18, 1990 , wherein an internal D.C. voltage due to dielectric anisotropy in an LCD is compensated and the driving is performed with a small amount of power consumption. Further, the driving method disclosed in this material teaches a feature of capability of preventing occurrence of light blinking (i.e., flicker) in a screen by inverting a polarity of an image signal voltage at every scan line on a display screen. This feature was disclosed in the Japanese Patent Publication Laid-Open Nos. 60-3698 (1985), 60-156095 (1985) and 61-275822 (1986).
In recent years, there have been strong demands for providing wider screens for active LCDs and the ability of displaying more minute images with high quality. The charging time is getting shorter as more minuteness is achieved. For example, in an LCD for use at a work station, about 1,000 scan lines are used and the charging time per one scan line is 17 .mu.seconds. This charging time is about one forth that of a small-sized liquid display television (TV) which has 240 scan lines and requires 60 .mu.seconds for charging. A wider screen and more minute images cause another problem, that is, delays in scan signals. Such shorter charging time and delays in scan signals cause gradient brightness at the right and left sides of a screen.
In the above-mentioned method of "capacitive-coupling drive", such a signal delay leads to serious troubles.
In this conventional driving method, a gate signal delay causes a charging error that a source signal transmitted via the image signal line cannot be transmitted to the pixel electrode within a gate-ON time period, or causes a time delay until the gate voltage reaches an off-level so that the established electric charge leaks, causing a change in the pixel potential.
The charging rate of a pixel signal is proportional to the following equation. EQU ((W/L*.mu.*Cox) * (Vg-Vs-Vth)) / Ct
where Ct=Cs+Cgd+Csd+Clc
Cs: storage capacitance PA1 Cgd: gate-to-drain capacitance PA1 Csd: source-to-drain capacitance PA1 Clc: liquid crystal capacitance PA1 W: channel width of TFT PA1 L: channel length of TFT PA1 .mu.: mobility PA1 Cox: gate capacitance PA1 Vg: gate voltage PA1 Vs: signal voltage PA1 Vth: threshold voltage of TFT
Among the above factors, Ct is determined by the specification of an LCD. .mu., Cox, Vth are virtually determined by the performance of a TFT. In the case of a small-sized LCD, a charge which exactly corresponds to a pixel signal can be established by setting W/L of a TFT to a large value.
Setting the W/L to a large value, however, makes the parasitic capacitance Cgd of the TFT large too, which increases the capacitance connected to the gate wiring, resulting in a considerable delay of gate signals. Therefore, there exists a value of W/L for maximizing the charging rate. With the value of W/L for maximizing the charging rate, a desired charging rate, however, cannot be achieved in a large-sized and high-precision LCD.
In order to solve the problem of charging errors, there has been proposed a method in the Japanese Patent Application Laid Open No. 3-222049, published on Oct. 1, 1991, in which a charge is preliminarily established to a vicinity of a desired voltage value by supplying a preliminary additional ON signal for precharge before supplying a substantial gate ON signal by 2 H ("H" represents a horizontal scanning period). However, it is not possible to obtain the effect of the preliminary charge by simply applying the above driving method to the aforementioned method of "capacitive-coupling drive" and, as a consequence, there occurs an error in the pixel potential.